The invention concerns a process for the elimination of waste water produced upon upon desulfurization of coking oven gas by means of a washing solution containing organic oxygen carrier, with simultaneous recovery of elemental sulfur. The waste water is decomposed in a combustion chamber in a reducing atmosphere at temperatures between about 1000.degree. and 1100.degree. C. under such conditions that the mol ratio of H.sub.2 S:SO.sub.2 amounts to at least 2:1 in the exhaust gas produced in the combustion chamber, sulfur present therewith is separated, the sensible heat of the exhaust gas is utilized for steam generation, and subsequently the cooled and desulfurized exhaust gas is added to the coking oven gas before the pre-coolers.
In practice, washing techniques for the desulfurization of the coking oven gas, i.e. mainly the removal of hydogen sulfide, have been predominantly selected which operate with organic oxygen carrier-containing, alkaline or ammoniacal washing solutions. As organic oxygen carrier, mainly quinone or compounds of quinoidal character, such as e.g. hydroquinone, naphthaquinone, anthraquinone, as well as their sulfonic acids, or also other organic compounds with an appropriate redox potential, are used. The processes usually are so performed that the hydrogen sulfide is initially washed out by means of the organic oxygen carrier-containing washing solution in a washer of suitable type of construction. The regeneration of the loaded wash solution then follows with air in so-called oxidizers, whereby the oxygen of the air oxidizes the hydrogen sulfide into sulfur and water by means of the oxygen carrier contained in the washing solution. The sulfur is then normally removed from the wash solution by heating it under pressure to temperatures above the sulfur melting point, whereupon the molten sulfur is separated in liquid state from the washing solution in a separating container, and the washing solution, freed of sulfur, after appropriate cooling is provided again in circulation to the hydrogen sulfide washer.
With such washing operations not only hydrogen sulfide but also hydrocyanic acid contained in the coking oven gas is washed out, and this acid is reacted with the elemental sulfur into rhodenides in the oxidizers. Moreover, with the oxidation of the hydrogen sulfide into sulfur there occur side reactions which lead to sulfates, thiosulfates and other sulfur compounds. In the course of time the wash solution led in circulation becomes enriched in these compounds, whereby the washing effect is impaired.
In order to avoid this enrichment, it is therefore necessary to discard a portion of the washing solution led in circulation, at determined time intervals, and to replace with fresh washing solution. This discarded washing solution cannot however be left in the drainage as waste water. On account of the substances contained in the discarded washing solution, the waste water is namely strongly toxic and oxygen-consuming, so that it requires initially an appropriate working up before it can be led to drainage.
Proceeding from this, DE-PS No. 27 55 830 already describes a process for the elimination of this waste water, in which the waste water to be treated is initially concentrated 70 to 30%, and subsequently is reacted in a combustion chamber under reducing atmosphere with the previously mentioned conditions. Coking oven gas and ammonia vapors resulting from the coking oven gas treatment are burned together in the absence of air to generate the reducing atmosphere.
Whether or not the process described in this reference has properly been proven to work, the present invention is based upon the object of still further improving this known manner of operation and simplifying the same with regard to apparatus. This applies in particular with regard to the previously necessary pre-concentrating of the waste water as well as the removal of sulfur from the loaded washing solution.
The process of the above-mentioned type serving for attainment of this object is characterized according to the present invention by the following particulars:
Sulfur in the washing solution is separated in the oxidizers and is led into the combustion chamber together with the portion of the washing solution which is separated from the washing solution circulation as waste water, whereby the sulfur loading of the waste water can amount initially up to about 370 kg sulfur per m.sup.3 and the cooling of the sulfur-containing exhaust gas leaving the combustion chamber follows in a waste heat boiler and a sulfur condenser heated with pre-heated boiler feed water, from which the sulfur is discharged in liquid state.
A pre-thickening of the waste water is thus avoided with the process according to the present invention, and at the same time the sulfur which is separated in the oxidizer is led together with the waste water into the combustion chamber. Not only the apparatus and process-technical expenditure for the precondensing of the waste water are avoided thereby, but also the apparatus expenditure for the sulfur recovery is reduced, since the otherwise following thermal separation of the sulfur/washing solution does not take place. A further advantage of the manner of operation according to the present invention is that a highly pure sulfur is obtained which possesses the characteristics of Claus sulfur.
Reaction of the mixture of sulfur and waste water in the combustion chamber follows preferably at a temperature of about 1050.degree. C., whereby the reducing atmosphere in the combustion chamber can be produced not only by combustion of the coking oven gas alone but also by coking oven gas together with the ammonia vapors produced with the coking oven gas treatment, under air exclusion. It is possible herewith for the exhaust air from the oxidizers as well as any exhaust air from the coking oven gas treatment to be led into the combustion chamber.
According to a preferred embodiment the process according to the present invention can moreover be performed in such manner that the sulfur-containing washing solution leaving the oxidizer is filtered to such an extent that yet so much washing solution adheres to the sulfur, present as filter cake, as to have to be disposed as waste water from the washing solution circulation. In this case the produced moist filter cake is led into the combustion chamber.
The novel features which are considered characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.